In the past, the methods used for the production of carbon black have largely been determined by the availability of inexpensive fossil fuels for energy and raw material requirements therein, and by the intended use of the carbon black product, whether as a pigment in ink or the like, or as a rubber-additive in the manufacture of automotive tires, etc. In recent years, the fossil fuels of choice, because of availability and price, have been primarily residual oils, petroleum tars, coal tars, and natural gas.
Three methods of producing carbon black have been dominant: the oldest of these, the channel process, is a low efficiency conversion method in which natural gas-fueled flames are impinged upon a relatively cool surface.
In the second method, the furnace combustion process (the most widely used at present), carbon black for the rubber industry is produced by the combustion and cracking of a mixture of oil and air. This process has a conversion efficiency about four to five times that of the channel process.
The third method, the furnace thermal process, provides in the neighborhood of 10% of the present production of carbon black. That process has a carbon conversion efficiency approximately twice that of the furnace combustion process. It involves the intermittent heating of a furnace filled with ceramic checkerwork to cracking temperatures. The furnace is heated by the combustion with air of natural gas or petroleum and/or hydrogen produced in the thermal decomposition of such materials to carbon black. The flow of air is then terminated without discontinuing the flow of fuel; thermal decomposition of the hydrocarbon fuel is thus effected, with the concomitant formation of carbon black.
The realization that the supply of gaseous and liquid fossil fuels is limited and that the cost of such fuels can never decrease, has spurred interest in research into methods for the economic production of non-polluting fuels from carbonaceous feedstocks such as coal, char, coke, natural gas, wood and other cellulosic materials, municipal solid wastes, and agricultural products. Most of such research has centered on the manufacture of substitute natural gas (SNG) and synthetic liquid hydrocarbon fuels from coals of various ranks. These processes invariably require the separate manufacture of large quantities of hydrogen required as a reactant.
The results of some of the foregoing research efforts have been described in the patent literature, e.g., in Schora U.S. Pat. No. 3,861,885, which describes a process for producing a pollutant-free carbon black solid fuel useful as feed for coal-fired turbines, as an additive to diesel fuel, and as a material for pipelining to areas where air pollution requirements dictate a fuel with low sulfur content. This process involves the initial pretreatment of coal to remove a hydrocarbon stream, followed by gasification of the resulting devolatilized coal to yield a product gas that is essentially carbon monoxide and hydrogen, followed by cooling the product gas under controlled conditions in a fluidized bed to precipitate carbon black in a finely divided state.
Johnson U.S. Pat. No. 3,424,556 discloses a method of producing carbon black from coal, involving the initial decomposition of the coal to tar, ash, and gaseous hydrocarbons, followed by dehydrogenation of the tar hydrocarbons and aggregation of the carbon. Cheng et al. U.S. Pat. Nos. 3,975,504; 4,072,468; and 4,206,175 similarly describe the formation of carbon black by utilizing hot combustion gases to cause pyrolytic decomposition of coal or a hydrocarbon feed, the hot combustion gases being produced by the oxidation of a carbonaceous fuel.
In present commercial processes for the manufacture of carbon black (thermal black) from natural gas, a substantial fraction of the carbon black produced is recovered from a hot hydrogen-rich gas phase by spray cooling with industrial water and collection of the cooled carbon black from the bottom of the spray cooling apparatus. The remainder of the carbon black product is recovered from the refractory-lined walls of the carbon black reactors. The carbon black thus produced tends to be contaminated with hardness from the cooling water employed, and possibly with other impurities adsorbed during collection from the spray-cooling equipment or during the subsequent separation from the cooling water, as well as with refractory material from the reactor walls.
The preceding, more recent examples of the prior art, all use an oxygen-containing gas to react with a carbonaceous material as a step in the production of particulate carbon black, in contrast to the former profligate, partial combustion and thermal decomposition of natural gas.
The preparation of methane by the hydropyrolysis of coal has also been disclosed in the prior art. Hence, Ullman et al. U.S. Pat. No. 4,597,776 describes the treatment of a hydrogen-deficient carbonaceous material with a hydrogen-containing pyrolysis gas at an elevated temperature and pressure to produce a product gas mixture comprising methane, carbon monoxide and hydrogen. This product gas mixture is enriched with a specified concentration of hydrogen by contacting it with a controlled amount of steam in a water-gas shift reaction zone to react at least a portion of the carbon monoxide to produce hydrogen. The resulting hydrogen-rich gas mixture is cryogenically separated into its constituent parts, and a mixture comprising hydrogen, carbon monoxide, and methane, is combined with a controlled amount of steam and recycled.
One of the present inventors has also suggested the preparation of pollutant-free particulate carbon by pyrolyzing coal or other carbonaceous material to methane, followed by thermally decomposing the methane to a particulate carbon. See "The Direct Use of Natural Gas (Methane) For Conversion of Carbonaceous Raw Materials to Fuels and Chemical Feedstocks", presented at the International Symposium on Hydrogen Systems, held in Beijing, China, on May 7-11, 1985 ("Hydrogen Systems", Vol. II, Edited by T. N. Veziroglu et al, pp. 217-228). The cited paper does not disclose the specific manner in which such a process may be carried out.
In parent application Ser. No. 54,610, filed May 27, 1987, the present inventors have disclosed a specific technique for combining the exothermic hydropyrolysis of carbonaceous materials to methane with the endothermic decomposition of the resulting methane to produce a pollutant-free particulate carbon. In the process described therein, the methane-rich gas stream produced by the hydropyrolysis reaction is de-oxygenated by a dewatering operation, preferably recuperatively wherein most of the thermal energy removed during pre-cooling of the humid gas prior to condensation is returned to the process during reheating of the dewatered gas, thereby facilitating high carbon conversion efficiencies in the subsequent methane pyrolysis reaction. By interposing the de-oxygenating step as described, a commercially feasible process for converting carbonaceous feedstocks to particulate carbon black is provided.
Alternatively, the hydrogen-rich gas stream exiting from the methane pyrolysis reactor can be similarly de-oxygenated to enhance carbon conversion efficiencies, but in the cases considered to date, deoxygenating the methane-rich gas exiting from the hydropyrolysis reactor is more effective. As discussed below in connection with the present invention, the reverse is true when deoxygenation is performed by means of the production and recovery of methanol (instead of by the condensation and withdrawal of water.)
The separate manufacture of hydrogen is not required in the process of the aforesaid parent application Ser. No. 54,610 since the ultimate primary product,particulate carbon, contains no hydrogen. Moreover, as described hereinafter, in the process of the present invention sufficient hydrogen and oxygen are frequently present in the carbonaceous feedstock being processed to permit the co-production of substantial quantities of the secondary co-product, methanol, without requiring the addition of extra hydrogen.
It is, therefore, among-the objects of the present invention to provide a further improved process for the conversion of carbonaceous feedstocks to substantially pollutant-free particulate carbon or carbon blacks. A further object of the invention is to provide such a process which directly produces, in addition to such carbon blacks, methanol, separately or in combination with the carbon blacks as a novel non-polluting liquid fuel.